How Do HEPA Filters Work?

HEPA filters are widely known for their exceptional ability to trap airborne pollutants. But what exactly makes them so effective? In this article, we’ll dive into the science behind HEPA filters work, and why understanding their filtration mechanisms is crucial for making the right HVAC or air purifier choice.

What Makes a Filter HEPA?

To be classified as a HEPA filter, it must capture at least 99.97% of particles as small as 0.3 microns—the size most likely to penetrate deep into the lungs. This efficiency standard is certified under international norms such as EN 1822 and ASME standards.

HEPA filters differ from standard filters in their fiber density and precision. Most are made from fiberglass or synthetic materials, tightly woven into mats that maximize particle capture while maintaining airflow.

The Science Behind HEPA Filtration

The working principle of HEPA filters revolves around airflow dynamics and particle behavior. Contrary to what many think, HEPA filters don’t function like sieves. Instead, they rely on a combination of three microscopic filtration mechanisms to trap particles of various sizes as they move through the filter medium.

The Three Mechanisms of Filtration

Interception

Particles that follow the air stream but come within one particle radius of a fiber are intercepted and stick to it.

• Effective For: Mid-size particles such as pollen or pet dander
• Visual Concept: A particle moving in a smooth line and lightly brushing against a fiber, getting caught

Impaction

Larger particles cannot easily change direction with the air stream due to inertia. These particles slam directly into the filter fibers.

• Effective For: Large particles like dust or mold spores
• Analogy: Like a heavy car unable to make a sharp turn, these particles veer off and crash into a barrier

Diffusion

Small particles, especially those less than 0.1 microns, are affected by collisions with air molecules and move erratically—this is known as Brownian motion.

• Effective For: Ultrafine particles, including smoke, bacteria, and some viruses
• Mechanism: Their random paths increase the chance they will collide with and stick to filter fibers

Filter Structure and Airflow Path

A typical HEPA filter consists of multiple layers of randomly arranged fibers. As air flows through the filter, the pressure drop across the media causes contaminants to interact with the fibers, enabling the three filtration mechanisms described above.

The efficiency of this system relies heavily on:

• Fiber density
• Air velocity
• Total surface area of the filter media

Efficiency and Limitations

While HEPA filters are incredibly efficient, they do have some limitations:

• Clogging: Accumulated particles can reduce airflow over time
• Humidity Sensitivity: High humidity may affect fiber integrity
• Pressure Drop: Can increase energy consumption if not maintained

To combat these, many HVAC systems use pre-filters (to trap larger debris) and post-filters (to neutralize odors or VOCs).

For tips on extending filter lifespan and maintaining optimal performance, see our HEPA Filter Maintenance Checklist.

Applications Where This Matters

HEPA filters are used in environments where clean air is essential:

• Healthcare: Operating rooms, isolation wards
• Pharmaceutical & Tech: Cleanrooms, manufacturing lines
• Commercial & Residential HVAC: Air purifiers, fresh air systems

To learn about Klima Global’s solutions for these sectors, check out our Ultimate Guide to HEPA Filters and Industrial Applications of HEPA Filters.

Conclusion

HEPA filters are marvels of engineering that use physics—not chemicals—to purify the air we breathe. By understanding how they work through interception, impaction, and diffusion, you can better evaluate the right filtration system for your home, business, or healthcare facility.

Explore more in our HEPA Filter Maintenance Checklist or return to the Comprehensive Guide to HEPA Filters for broader insights.